1. Field of the Invention
The present invention relates generally to the field of speed reducers, such as single and multiple-stage gear reduction machines. More particularly, the invention relates to a technique for manufacturing a plurality of worm shafts having different thread patterns corresponding to different gear ratios using a single blank model.
2. Description of the Related Art
Gearboxes are commonly employed to convert the rotational speed of a rotatable output shaft of a device to a lower, or higher, rotational speed that is more desirable for a given application. Gearboxes receive a shaft driven at a first rotational speed and use gears within the gearbox to drive an output shaft at a second speed. The ratio of the input speed to the output speed is known as the gear ratio. When the second speed is slower than the first speed, the gearbox is known as a speed reducer and the gear ratio is commonly referred to as the reduction ratio. Speed reducers are commonly employed when the prime mover, such as an electric motor, operates more efficiently at a high speed, while a lower speed is more desirable for the application. Speed reducers in industrial applications may include single or multiple stages depending upon the speed and torque requirements and the overall gearbox ratio. Moreover, in most applications the gear ratio of the speed reducer is fixed, with changes in input-to-output ratios being varied by varying the input speed, by the use of a transmissions, a variable-speed sheave drive, and so forth.
Worm gears are an example of the types of gears that have been used in gearboxes, including speed reducers. A worm gear typically consists of a spirally, or helically, threaded shaft, referred to as the worm shaft, and a toothed wheel, referred to as the gear. The worm shaft is used to drive the gear. In single-stage speed reducers, the worm shaft is typically coupled to an input shaft, which may be coupled to a drive shaft of a prime mover. The gear is coupled to an output shaft, which, in turn, may be coupled to a load. The worm shaft meshes with the gear and causes the gear to rotate as the worm shaft is rotated. The worm shaft rotates about a first axis and the gear rotates about a second axis. The displacement between the centers of these two axes is known as the center spacing. The gear ratio is defined by the parameters of the worm shaft and the gear in accordance with well-established formulae. In multiple-stage gear reduction sets, multiple worm gears are used to successively reduce the speeds of rotation to obtain a desired output speed for the load. The overall gear reduction ratio is a product of the reduction ratios of the successive stages.
A speed reducer typically is positioned between a drive shaft of a motor and an input shaft of a load. The worm shaft typically is driven by the drive shaft of the motor and the input shaft of the load is driven by the output shaft of the gearbox. Thus, alignment of the drive shaft with the worm shaft and the load with the output shaft is important for proper operation. Therefore, worm gear gearboxes typically are characterized by their center spacing, or frame size. The frame size of a speed reducer corresponds to the center spacing between the center of the axis of the worm shaft and the center of the axis of the gear. For a given frame size, speed reducer manufacturers typically produce a family of speed reducers, each having a different reduction ratio.
Different worm shafts and gears are used to produce the different reduction ratios. Generally, the worm shafts are designed and manufactured separately according to the desired reduction ratio and the size of the frame of the speed reducer. Typically, each worm shaft is machined from a blank worm shaft. However, because the worm gears are designed and manufactured separately according to the desired reduction ratio and the size of the frame of the speed reducer, different blanks are used to manufacture different worm shafts having different thread patterns, even when they are to be used in the same frame. This increases the number of different parts that a worm gearbox must maintain in inventory.
There is a need, therefore, for an improved technique for manufacturing worm shafts which would reduce the overall number of different parts that are used to manufacture a family of speed reducers.
The present technique provides a novel approach to manufacturing worm shafts and gearboxes to respond to these needs. In accordance with the first aspect of the technique, a method is provided for manufacturing a worm shaft for use in a family of gearboxes. The method may comprise establishing a desired axial pitch for a thread of the worm shaft to produce a desired gearbox ratio. The desired axial pitch may be based on the outer diameter of a worm shaft blank used for all gearbox ratios of the family of gearboxes. The method also may comprise adapting the worm shaft blank with a thread having the desired axial pitch.
In accordance with another aspect of the present invention, a family of gearboxes is featured. Each gearbox in the family of gearboxes comprises a worm shaft, a gear, and a frame. The worm shaft and gear are adapted for engagement to produce rotation in the gear. The frame is adapted to support the worm shaft and gear. The worm shaft and gear are supported by the housing to a specific center spacing between the worm shaft and the gear. The family of gearboxes is defined by the center spacing of the worm shaft and the gear and includes speed reducers of a plurality of different gearbox ratios. Each worm shaft in the family of gearboxes has substantially the same outer diameter of the thread.